Method for regulating membrane moisture of a polymer electrolyte fuel cell, and a polymer electrolyte fuel cell

ABSTRACT

Fuel cell stack and method of regulating membrane moisture of polymer electrolyte membranes of fuel cells of the fuel cell stack includes ascertaining electronically an average value of an electrical value corresponding to moisture of the polymer electrolyte membranes of a number of fuel cells of the fuel cell stack without utilization of an auxiliary electrode, the number of fuel cells ranging from two fuel cells to all fuel cells of the fuel cell stack; and adjusting the moisture of the polymer electrolyte membranes of the number of fuel cells to an optimum moisture as a function of the average value ascertained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of regulating the membrane moisture ofa polymer electrolyte fuel cell, and to a polymer electrolyte fuel cellcomprising a means for regulating the membrane moisture. The fuel cellscontain solid polymer membranes as electrolyte and preferably usehydrogen as burnable gas and air or oxygen under low pressure asoxidizing agent.

2. Description of the Related Art

Polymer electrolyte membrane fuel cells, as they are commonly employedfor producing electric current, contain an anode, a cathode and an ionexchange membrane disposed therebetween. A plurality of fuel cellsconstitutes a fuel cell stack, with the individual fuel cells beingseparated from each other by bipolar plates acting as currentcollectors. For generating electricity, a burnable gas, e.g. hydrogen,is introduced into the anode region, and an oxidizing agent, e.g. air oroxygen, is introduced into the cathode region. Anode and cathode, in theregions in contact with the polymer electrolyte membrane, each contain acatalyst layer. In the anode catalyst layer, the fuel is oxidizedthereby forming cations and free electrons, and in the cathode catalystlayer, the oxidizing agent is reduced by taking up electrons. Thecations migrate through the ion exchange membrane to the cathode andreact with the reduced oxidizing agent, thereby forming water whenhydrogen is used as burnable gas and oxygen is used as oxidizing agent.In the reaction of burnable gas and oxidizing agent, there are releasedconsiderable amounts of heat that must be dissipated by cooling. Coolingso far has been achieved by cooling channels in the bipolar platesthrough which deionized water is flown.

With this kind of cooling, tremendous material problems result sincethere are typically about 50 to 300 bipolar plates connected in series,with the cooling water thus electrically joining together differentpotentials. The result thereof are material decompositions. Inaccordance therewith, solely graphite or gold-plated metal are feasibleas material for the bipolar plates.

Furthermore, it is necessary to keep the polymer membrane moist, sincethe conductivity value of the membrane is greatly dependent on its watercontent. To prevent drying up of the membrane, there was thus required acomplex system for humidifying the reaction gases.

It is the object of the invention to make available a polymerelectrolyte fuel cell and a polymer electrolyte fuel cell stack,respectively, in which the polymer electrolyte membrane of a fuel cellhas the optimum moisture content at all times during operation.

An additional object of the invention consists in making available amethod which renders possible to keep the polymer electrolyte membraneof a polymer electrolyte fuel cell at an optimum moisture content duringoperation of the fuel cell.

SUMMARY OF THE INVENTION

The object is met by the method of regulating the membrane moisture of apolymer electrolyte fuel cell according to claim 1, the polymerelectrolyte fuel cell according to claim 7 and the fuel cell stack of aplurality of polymer electrolyte fuel cells according to claim 12.

Preferred developments of the invention are indicated in the dependentclaims.

Polymer electrolyte membranes require a high water content to ensureoptimum conductivity for H⁺ ions. The water content must be maintainedas a rule by supply of water, as otherwise the burnable gas flows andoxidizing agent gas flows flowing through the cell dry up the membrane.However, to counteract possible drying up by the addition of an excessof water, is not sensible since water in too large quantities results inflooding of the electrodes, i.e. the pores of the electrodes areclogged. Simple ascertaining and regulating the particular amount ofwater required has not been possible so far.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a preferred embodiment of a fuel cell according to theinvention,

FIG. 2 shows a circuit for measuring the impedance of fuel cells,

FIG. 3 shows the dependency of the conductivity a NAFION® membrane onthe water content of a membrane,

FIG. 4a shows a schematic representation of the control of wateraddition,

FIG. 4b shows a schematic representation of controlling a change of theoperating conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A polymer electrolyte fuel cell according to the invention uses air oroxygen at slight overpressure as oxidizing agent. Preferred is anoverpressure of less than 2 bar, with an overpressure of less than 0.5bar being particularly preferred. The necessary pressure difference canalso be obtained by suction. As burnable gas, preferably hydrogen isused, but the use of other burnable gases is in principle possible waswell. As polymer electrolyte membrane, preferably NAFION® is employed,hydrogen is supplied to the individual fuel cells of a stack anddistributed via gas channels in the anode region. Air is supplied at thesame time and distributed via gas channels in the cathode region. Thehydrogen migrates to the anode catalyst layer and forms cations therewhich migrate through the electrolyte, a proton exchange membrane, tothe cathode. At the cathode, oxygen migrates to the cathode catalystlayer and is reduced there. During the reaction with the cations, wateris created as reaction product. Due to the reaction heat, the waterformed evaporates, which results in a certain cooling effect. Thiscooling effect, however, is not sufficient on the one hand, and on theother hand the membrane in the course of operation of the fuel cell isincreasingly depleted of humidity.

As can be seen from FIG. 3 for NAFION® NE 105 (30° C.), the conductivityof ion-conducting membranes increases with the H₂O content.N(H₂O)/N(SO₃H) designates the number of water molecules per sulphonicacid remainder of the membrane.

A reduction of the moisture content of the solid polymer electrolytemembrane of a fuel cell thus has the consequence that its internalresistance increases, i.e. that its conductivity value decreases. Theconductivity value of the membrane is dependent in extreme manner on itswater content. What is essential for efficient operation of a polymerelectrolyte fuel cell is thus that the polymer electrolyte membrane atall times have the optimum humidity corresponding to the particularoperating conditions (temperature, load, air ratio).

For maintaining optimum humidity, it is possible according to theinvention to determine during operation of the fuel cell, preferablyregularly or continuously, whether the membrane is moist in optimummanner or whether measures for adjusting the optimum membrane moistureneed to be taken.

Adjusting the optimum membrane moisture can take place, for example, byadding the required amount of water in the liquid or gaseous state.Preferably, the water is added to one of the reaction gases or bothreaction gases. Additional possibilities of keeping the membrane at anoptimum moisture content consist in matching the operating conditions tothe moisture content ascertained. The operating conditions mainlyconceivable in this respect are the electrode temperatures, the volumeflows of the reaction gases and the load of the fuel cell. According tothe invention, setting the optimum membrane moisture thus takes place asfollows: after ascertaining to what extent the actual moisture differsfrom the desired moisture, a specific amount of water is added or thetemperature of at least one of the electrodes or the volume flow ofburnable gas and/or oxidizing agent is altered in such a manner that theactual moisture of the membrane corresponds to the optimum membranemoisture at said changed operating conditions. Changing of the load ispossible as well to achieve in essence conformity between the optimummembrane moisture and the actual membrane moisture. According to theinvention, it is also possible to combine several ones of theabove-mentioned measures in order to effect setting of the optimummembrane moisture.

In the following, the invention will be elucidated by way of regulatingthe water addition.

The amount of water added basically can vary very much. It is dependenton the particular operating conditions of the fuel cell, and it isdependent in particular also on the type of cooling of the fuel cell.Frequently, fuel cells are fed with water for cooling which, dependingon the construction of the fuel cells, humidifies to a certain extentalso the membrane. As a rule, less additional water has to be suppliedthen than in case of cells employing exclusively air cooling, forexample.

The conductivity value of the membrane depends on the water contentthereof. During operation of a fuel cell, however, the conductivityvalue of the membrane cannot be measured directly. According to theinvention, a measure of the membrane moisture, preferably the impedanceof the fuel cell (value of the impedance or particularly preferred, thereal part of the impedance) is ascertained. Since the conductivity valueof the membrane is a continual, monotonic function of these quantities,the necessary amount of water can also be regulated on the basis of theimpedance.

A possible circuit for measuring the impedance of a fuel cell is shownin FIG. 2. As shown in FIG. 2, the fuel cells 1 comprise an anode region3, a cathode region 2, a polymer electrolyte membrane 4 disposed therebetween, and bipolar plates 10, 6 at the anode side and at the cathodeside, respectively. The anode-side bipolar plate simultaneously is thecathode-side bipolar plate of a neighboring cell. The anode and cathoderegions are designed as different layers carrying a suitable catalyst(without reference numeral) adjacent to the membrane.

The determination of the measure of the membrane moisture according tothe invention does not necessitate auxiliary electrodes, i.e. it makesuse of the operating electrodes. Intervention or interference with thecell proper is not necessary.

Direct measurement of the conductivity value and thus of the moisturecontent of a polymer electrolyte membrane of a fuel cell bydetermination of the impedance is carried out by modulation of the cellvoltage with an alternating signal having a frequency of 1 to 20 kHz. Incase of a fuel cell stack, suitably the average moisture content ofseveral membranes is measured. The quotient of alternating voltage andthe resulting current response is a measure for the moisture. In FIG. 2,BZ represents the fuel cell and R_(L) represents the load resistor.Connected in parallel to the load resistor is an assembly of capacitorC, resistor R and alternating voltage source U, which is suitable toproduce small alternating voltages (in the order of magnitude of about10 mV) and large currents (in the order of magnitude of about 10 A). Thevoltage of the fuel cell is modulated by the alternating signal (about 1to 20 kHz). The alternating voltage component U effects an alternatingcurrent I to be superimposed on the fuel cell current. The quotient ofalternating voltage and alternating current is a measure for theimpedance of the fuel cell and thus a measure for the moisture of thepolymer electrolyte membrane and for the necessary amount of water to beadded, respectively.

However, the amount or value of the impedance is dependent, in additionto the conductivity of the membrane, on further determinativequantities, namely on the size of the catalyst surface in contact withthe membrane, the ohmic resistance of the electrodes and the poisoningof the membrane by foreign ions. These quantities are subject to acertain amount of change in the course of the service life of a fuelcell, with the deviations due to change of the ohmic resistance of theelectrodes and due to poisoning of the membrane by foreign ions being asa rule negligible. In the course of the life of a fuel cell, the valueof the impedance which corresponds to the optimum membrane moistureunder the given operating conditions (desired or set value of the amountof the impedance), can thus vary. Thus, the desired value to be observedof the amount of the impedance should be set each time anew in thecourse of arising maintenance work. In doing so, the new desired valueis determined by maximizing the performance of the fuel cell. Duringoperation of the fuel cell, the optimum desired value can be matchedanew in alternative manner by Fuzzy logic or other methods familiar tothe expert, in accordance with the changed conditions.

A measure for the conductivity of the membrane that is largelyindependent of the catalyst surface (whose change in essence isresponsible for the change of the desired value of the impedance) isobtained if, in addition to the amount of the impedance, its phase angleis considered as well. If the real part of the impedance determinedelectronically therefrom is regarded as regulating variable, one soledesired value can be employed even over the entire service life of thefuel cell.

During operation of the fuel cells, the impedance (amount or real part)can be measured continuously or at regular intervals. In case a too lowconductivity value of the membrane or membranes is calculated on thebasis of the measurement, water is supplied to the system, for exampleby electronically controlled opening of water inlet valves, as is usual,until the desired value of the impedance is reached again, or one ormore of the operating conditions are varied in corresponding manner.

In case of fuel cell stacks with a plurality of fuel cells, it isfavorable not to determine the amount or the real part of the impedancefor each membrane individually, but to determine average values for aplurality of cells of the stack or even for all cells of the stackjointly and to arrange the necessary addition of water in accordancetherewith.

FIG. 4a schematically illustrates a specific example of controlling theintroduction of membrane humidifying water into a fuel cell stack 20operated with hydrogen 21 and air 22. In case too low membrane moistureis measured, water from water supply tank 23 is fed via valve 25 intothe hydrogen flow 21 until the required membrane moisture is reached.opening and closing of valve 25 is effected by control apparatus 24.

FIG. 4b schematically illustrates a specific example of controlling thechange, of operating conditions (air volume flow and load) in a fuelcell stack 20 operated with hydrogen 21 and air 22. In case a membranehumidity value is measured that is not optimum, control apparatus 30controls, as required, increased opening or closing of valve 31 untilthe necessary air volume flow has been obtained. As an alternative, itis also possible to change the hydrogen volume flow 21 or both flows. Inparticular with air-cooled fuel cells, control of the temperature ispossible as well via control of the air flow 22. Another alternativeresults by an air-flow independent variation of the temperature of thefuel cell (e.g. in case of water-cooling), which also effects avariation in moisture content of the membrane. Another possibilityconsists in controlling the load 36 by means of the control apparatus35.

Irrespective of the manner of determination of the optimum water contentof the membrane and the regulation of the water introduction, it ispossible according to the invention to use membrane humidifying watersimultaneously for cooling the fuel cell and for thus ensuringsufficient cooling. This is achieved according to the invention in thatin case of a fuel cell designed as outlined hereinbefore, ion-free waterin liquid form is introduced directly into the gas channels for thecombustion air. As an alternative, the water can also be introduceddirectly into the gas channels for the burnable gas.

A proven solution is the introduction of water both in the cathoderegion and in the anode region, particularly with operating conditionscausing severe drying up of the membrane.

The liquid water evaporates in the hot fuel cell and effects efficientcooling of the cell due to the phase conversion taking place.Furthermore, it penetrates into the polymer electrolyte membrane andkeeps it moist.

The easiest possibility of adding the necessary amount of water to theair flow and to the air and/or hydrogen flow, respectively, consists inintroducing the water into the gas channels by means of a metering pump,in numerous thin lines, e.g. capillaries. In doing so, no substantialmixing of the water with the air and the burnable gas, respectively,takes place, so that the free water surface available for evaporation isrelatively small.

A considerably larger free water surface and thus faster humidifying ofthe membrane and more efficient cooling is obtained if the requiredamount of water is added to the reaction gas flows in mixed form, i.e.as aerosol. The water-in-air aerosol and, if applicable, thewater-in-burnable gas aerosol contain water in the form of droplets witha size of 2 to 20 μm, which ensure rapid vaporization or evaporation.The aerosol can be produced for instance with the aid of ultrasonicatomizers or nozzles. The simplest production of the aerosol, which atthe same time requires the least amount of energy, takes place by meansof ultrasonic atomizers at frequencies of at least 100 kHz.

A particularly advantageous embodiment of the invention consists indesigning the passages or channels for receiving the water-in-airaerosol and the water-in-burnable gas aerosol, respectively, as shown inFIG. 1. In a fuel cell stack, each fuel cell is confined on the anodeside and on the cathode side by a bipolar plate 10, 6 each. Theanode-side bipolar plate simultaneously is the cathode-side bipolarplate of a neighboring cell and the cathode-side bipolar plate at thesame time is the anode-side bipolar plate of the other neighboring cell.

The bipolar plate, at least in a partial region, is of corrugated sheetstructure, i.e. it has alternating elevations and depressions. A surfaceof the bipolar plate 6 contacts, with its elevations 7, the cathoderegion 2 of the fuel cell, whereby the depressions 8 located between twoadjacent elevations each together with the cathode region form channels5 for receiving water-in-air aerosol 17. In corresponding manner, thebipolar plate 10 contacts with one surface the anode region 3 of thecell, so that the depressions 12 located between two adjacent anode-sideelevations 11 each also form channels 9 together with the anode region3. These can serve for taking up water-in-burnable gas aerosol.

In the embodiment shown in FIG. 1, hydrogen as burnable gas isintroduced perpendicularly to the plate surface through bores. Thehydrogen first enters channel 9 in communication with the supply openingand from there diffuses or flows into the adjacent porous anode region.From there, the hydrogen diffuses in part to the anode catalyst layerand in part into additional gas channels 9 in the plane of the anoderegion. Because of the outstanding diffusion properties of hydrogen, theentire anode region thus is uniformly supplied with hydrogen without aproblem.

If cooling water is to be supplied as well along with the burnable gas,it is as a rule more advantageous to choose the same type of feeding asin the cathode region, i.e. to feed fuel and water into each individualchannel 9. Because of the poor diffusion properties of water incomparison with hydrogen, only little water would penetrate the anodeotherwise, and the cooling effect would thus be low.

The construction has no separate cooling channels whatsoever. A specificadvantage consists in particular in that the path of the aerosol throughthe channels 5 of the cell constitutes a straight line. The corrugatedsheet structure of the bipolar plate with straight gas paths permits tominimize depositions of the aerosol and to conduct the necessary volumeflows with low pressure drop.

Flooding and clogging of the water-conducting paths by water droplets,as is frequently the case with porous plates, does not take place.Besides, the “corrugated sheet plate” can be manufactured veryinexpensively and simply in terms of manufacturing technology.

The anode and cathode regions are each designed as diffusion layerscarrying a suitable catalyst and disposed on the opposite sides of thepolymer electrolyte membrane 4.

Air gaskets 15, 15′ and hydrogen gaskets 16, 16′ seal the cell ingastight manner.

To increase the dwell period of the water in the cell and to thus enablecomplete evaporation, the walls of the gas channels 5 and/or the gaschannels 9 can be coated with a hydrophilic absorbent layer, forinstance with felt. The hydrophilic, absorbent layer distributes theintroduced amount of water in particularly even manner and retains thesame up to evaporation.

The amount of water required for obtaining optimum membranehumidification, as outlined hereinbefore, can be determined andregulated electronically. The amount of water introduced into the fuelcell can fulfill two tasks: cooling the cell and humidifying themembrane. For regulation of the required amount of water, however, onlythe setting of the suitable membrane moisture is taken intoconsideration. Depending on the parameters temperature, load, air ratioand the like, the optimum membrane moisture and thus the optimumconductivity value of the membrane is determined experimentally. Theaddition of water varies depending on the conductivity value to bereached. The cell temperature varies within wide limits depending on theoperating conditions. As long as sufficient water is introduced toensure optimum membrane moisture, a sufficient cooling effect, however,is ensured as well.

For keeping the moisture content of the reaction gases and thetemperature thereof along the direction of flow as constant as possiblein a fuel cell or fuel cell stack, the reaction gas, in particular theair, may be caused to pass the cell stack several times. This takesplace by recirculation of the air/water mixture leaving the fuel cellsand the burnable gas/water mixture leaving the fuel cells, respectively,to the respective suction or intake flow.

Thus, it is possible according to the invention in a polymer electrolytefuel cell, by introducing ion-free water in liquid form directly intothe gas channels of the combustion air and/or the burnable gas, toensure at the same time keeping of an optimum membrane moisture and,thus, an optimum conductivity value of the membrane as well assufficient cooling of the fuel cell.

What is claimed is:
 1. A method of regulating membrane moisture ofpolymer electrolyte membranes of fuel cells of a fuel cell stack,comprising: ascertaining electronically an average value of anelectrical value corresponding to moisture of the polymer electrolytemembranes of a number of fuel cells of the fuel cell stack withoututilization of an auxiliary electrode, the number of fuel cells rangingfrom two fuel cells to all fuel cells of the fuel cell stack; andadjusting the moisture of the polymer electrolyte membranes of thenumber of fuel cells to an optimum moisture as a function of the averagevalue ascertained.
 2. The method according to claim 1, wherein theaverage value ascertained electronically is an average of the electricalvalues corresponding to moisture of the polymer electrolyte membranes ofall fuel cells of the fuel cell stack.
 3. The method according to claim1, wherein ascertaining is carried out by modulating voltage of thenumber of fuel cells with an alternating signal.
 4. The method accordingto claim 3, wherein the electrical value corresponding to moisture ofthe polymer electrolyte membranes of a number of fuel cells of the fuelcell stack ascertained is impedance.
 5. The method according to claim 1,wherein adjusting the moisture of the polymer electrolyte membranes ofthe number of fuel cells to an optimum moisture is carried out by atleast one of (a) introducing water in a required amount, (b) changingelectrode temperature, (c) changing gas volume flow of at least reactiongases, and (d) changing load.
 6. The method according to claim 1,wherein the polymer electrolyte membranes are humidified and the fuelcells simultaneously cooled by introducing water in liquid form and in arequired amount directly into at least one of (a) gas channels for air,and (b) gas channels for burnable gas.
 7. A fuel cell stack, comprising:a plurality of polymer electrolyte fuel cells each having an anoderegion, a cathode region, and a polymer electrolyte membrane disposedbetween the anode region and the cathode region; means for supplying airas oxidizing agent to respective cathode regions; gas channel means fordistributing the air in the respective cathode regions; means forsupplying burnable gas to respective anode regions; gas channel meansfor distributing the burnable gas in respective anode regions;electronic means for ascertaining an average value of an electricalvalue corresponding to moisture of the polymer electrolyte membranes ofa number of fuel cells of the fuel cell stack without utilization of anauxiliary electrode, the number of fuel cells ranging from two fuelcells to all fuel cells of the fuel cell stack; and means for adjustingthe moisture of the polymer electrolyte membranes of the number of fuelcells to an optimum moisture as a function of the average value of theascertained.
 8. The fuel cell stack according to claim 7, wherein theelectronic means for ascertaining the average value of an electricalvalue corresponding to moisture of the polymer electrolyte membranes ofa number of fuel cells of the fuel cell stack ascertains the averagevalue of all of the fuel cells.
 9. The fuel cell stack according toclaim 7, wherein the electronic means for ascertaining the average valueof an electrical value corresponding to moisture of the polymerelectrolyte membranes ascertains the average value by modulating voltageof the number of fuel cells with an alternating signal.
 10. The fuelcell stack according to claim 9, wherein the electronic means forascertaining the average value of an electrical value corresponding tomoisture of the polymer electrolyte membranes ascertains impedance ofthe number of fuel cells.
 11. The fuel cell stack according to claim 7,wherein the means for adjusting the moisture of the polymer electrolytemembranes of the number of fuel cells to an optimum moisture as afunction of the average value ascertained is carried out by at least oneof (a) introducing water in a required amount, (b) changing electrodetemperature, (c) changing gas volume flow of at least reaction gases,and (d) changing load.
 12. The fuel cell stack according to claim 7,further comprising means for introducing water in liquid form directlyinto at least one of (a) the gas channels for air in the respectivecathode regions and (b) the gas channels for burnable gas in therespective anode regions, and bipolar plates positioned to confine therespective polymer electrolyte fuel cells on at least one of the anodeside and the cathode side.